There has been proposed the manufacture of composite articles comprising a boride of a first metal selected from the metals of Group IVb, Vb or VIb of the periodic table and a metallic phase constituted by a second metal. The manufacture can be by reaction sintering of a particulate mixture of precursors comprising said first metal or its hydride and a boride of said second metal.
U.S. Pat. No. 4,514,628 discloses a reaction sintered composite cermet material for use in aluminum production cells. The described material may comprise, e.g., TiB.sub.2 as the ceramic phase and aluminum as the metallic phase, produced by reaction sintering a precursor mixture of TiH.sub.2 and AlB.sub.2.
The kind of material disclosed in the above patent is reported to be wettable by molten aluminum and is therefore favorably usable in electrolytic aluminum production cells, where it may replace conventional carbon components which are not wettable by molten aluminum. This will permit a decrease in the anode-cathode distance due to the reduction of the depth of the electrowon aluminum for example in a drained cathode cell as described in U.S. Pat. No. 4,093,425 or in a packed bed cell as described in BP-B-0 033 630.
However, the beneficial effects of a reduction of the anode-cathode distance, resulting in energY savings must not be traded off by the higher cost of the new material. It is therefore a subject of ongoing efforts to decrease the production cost of this new type of proposed material.
An important factor for the price of these ceramic/metal composites is the duration of the heat cycle, i.e., the time required for the heating of the precursor material until completion of the reaction. Efforts have been made to substantially decrease this duration; however, such efforts were impaired by the formation of undesired reaction products during the sintering reaction.
It is another aspect of the prior art open for improvements that only relatively small shapes can be formed during one individual sintering process. Several of these small elements had to be assembled and, e.g., reaction bonded using a technique such as described in U.S. patent application Ser. No. 620,034, (EP0164830) to form an article of the desired size. This drawback is due to the limited admissible volume of the reaction mixture producing exothermic heat during the sintering reaction. When the volume of the reaction mixture was too high, too much heat was produced leading to unacceptable defects in the microstructure of the reaction product. Such defects may be a very coarse grain structure, as well as including cracks, non-uniform distribution of the ceramic phase within the metal phase and loss of metal phase. Such microstructural defects represent sites for mechanical failure in service.